The unique ability of certain thiamine dependent enzymes to convert aromatic nitroso compounds to hydroxamic acids is under investigation. So far two types of hydroxamic acids have been produced by enzymatic reactions with nitrosobenzene. These are the N-acetyl and N-glycolyl derived hydroxamic acids from pyruvate decarboxylase and transketolase, respectively. Although a mechanism for this reaction has been proposed, further work is in progress to prove this mechanism. The major emphasis of the research is now on in vivo conversions of nitroso compounds to hydroxamic acids.